Hot-working tools are required to have toughness to endure impacts since they are used in contact with a hot-temperature workpiece and a hard workpiece. Conventionally, alloy tool steels such as SKD61, which is a JIS steel grade, have been used for hot-working tool materials. Moreover, in response to recent demands for further improvement in toughness, alloy tool steel materials having an improved component composition of the SKD61 alloy tool steel have been proposed for the hot-working tool material (see Patent Literatures 1, 2).
Typically, a hot-working tool is fabricated by machining a hot-working tool material, which is in an annealed state and has a low hardness, into a shape of a hot-working tool, and thereafter subjecting it to quenching and tempering to adjust it to have a predetermined hardness for use. Moreover, after the adjustment to the above described hardness for use, the hot-working tool is typically subjected to finish machining. In some cases, the above described hot-working tool material is first subjected to quenching and tempering (formed into a state of so-called pre-hardened material), and thereafter is subjected to machining into a shape of the hot-working tool in junction with the above described finish machining. Quenching is an operation in which the hot-working tool material in an annealed state (or the hot-working tool material after it is machined) is heated to and held in an austenite temperature region, and thereafter rapidly cooled to cause its structure to transform into martensite. Therefore, the component composition of the hot-working tool material is adjusted such that it can obtain a martensitic structure by quenching.
Thus, in the martensitic structure after quenching, grain boundaries of austenite crystal which have been produced in the process of heating and holding the material to and in the above described austenite temperature region are recognizable as “prior austenite grain boundaries”. The distribution state of the “prior austenite grain diameter” formed by the prior austenite grain boundaries is substantially maintained even in the metal structure after subsequent tempering (that is, the structure of a completed hot-working tool).
Meanwhile, in an aspect of such hot-working tool, it is known that the toughness of the hot-working tool can be improved by reducing the contents of inevitable impurities contained in its component composition, such as P, S, O, and N. Amongst those, P segregates at prior austenite grain boundaries of the martensitic structure after quenching and tempering, thereby embrittling the grain boundaries and significantly reducing the toughness of the hot-working tool. Thus, a hot-working tool material (that is, a hot-working tool) in which P content is limited to not more than 0.020 mass % has been proposed (Patent Literature 3). It is also known that the toughness of a hot-working tool can be improved by reducing prior austenite grain diameter in the above described martensitic structure (Patent Literature 3).